Currently, there is no effective treatment for the chronic progressive phase of MS, when serious disability is most likely to appear.

Researchers cut the risk of nerve degeneration in mice with MS-type symptoms by giving them a form of vitamin B3 called nicotinamide.

The Children's Hospital Boston study appears in the Journal of Neuroscience.

MS, which affects about 85,000 people in the UK, is a disease of the central nervous system.

It causes the break down of the myelin sheath, a fatty protein, which coats nerve fibres, disrupting the ability to conduct electrical impulses to and from the brain.

Many patients develop a form of the disease called relapsing-remitting MS, in which bouts of illness are followed by complete or partial recovery. In this early phase anti-inflammatory drugs can help.

But eventually patients can enter the chronic progressive phase, for which there is no good treatment.

Women are twice as likely to be affected by MS as men

The Boston team worked on mice with an MS-like disease called experimental autoimmune encephalitis (EAE).

They found that daily nicotinamide shots protected the animals' nerve cells from myelin loss, and stabilised the condition of those cells that had already been affected.

The greater the dose of nicotinamide, the greater the protective effect.

Rating disability on a scale of one to five, mice receiving the highest doses of nicotinamide scored between one and two, while animals who received no shots at all scored between three and four.

Key chemical

The researchers found that nicotinamide boosted levels of a crucial chemical called nicotinamide adenine dinucleotide (NAD) in the animals' nervous systems.

Nicotinamide also significantly reduced neurological deficits even when treatment was delayed until 10 days after the induction of EAE - raising hope that it will also be effective in the later stages of MS.

Lead researcher Dr Shinjiro Kaneko said: "The earlier therapy was started, the better the effect, but we hope nicotinamide can help patients who are already in the chronic stage."

The researchers said nicotinamide was cheap, and thought to have few side effects.

However, they said further work was needed to test its effect on humans.

"This is interesting early research which we should like to see developed, adding our usual caution that what works in mice does not always work in men."

A spokesperson for the MS Trust said: "These are interesting results, but studies in mice with the experimental equivalent of MS may not necessarily translate into a successful treatment for people with MS."